proximity effect

The term proximity effect is used for an effect on Lyman-alpha forests (Ly-α forests) evident in EMR from quasars. The effect reveals quasar characteristics including hints regarding its history, specifically when it has been active and when it has not. While the quasar is shining, its ionizing radiation ionizes intergalactic medium (IGM) in the general vicinity of the galaxy and the ionized hydrogen (HII) does not absorb Lyman alpha (Ly-α), a fact observable in a quasar's Ly-α forest (numerous Ly-α absorption lines in EMR from the quasar, due to high-density regions of HI along the way, i.e., at various redshifts). The region surrounding the quasar where this occurs is termed the quasar's proximity zone. The effect has the potential of showing how long the quasar has (had) been shining, revealing some indication of the age of the quasar at that time, and how long the supermassive black hole (SMBH) has been there. It also has the potential of showing earlier variation of the brightness of the quasar as a function of time.

For pairs of quasars near each other across the celestial sphere, there is also potential also to draw conclusions from the effects of the nearer quasar's proximity zone on the Ly-α forest of the further quasar.

The analogous affect also occurs with IGM helium and the He II Ly-α line (any atom ionized so as to have just one electron has an analogous series to the hydrogen line series such as the Lyman series, that are at somewhat-different wavelengths, and He II produces a corresponding Ly-α forest).

Note that the term proximity effect has other uses in physics, such as in quantum theory.